Optical modulators have been employed for many years in the field of optical communications to accept modulated data in electrical format (typically radio frequency or RF) and transfer the data onto an optical carrier. In a Mach-Zehnder optical modulator 20, as generally shown in FIG. 1 (PRIOR ART), a beamsplitter 22 divides the laser light from an input optical waveguide 24 into two optical beams propagating in parallel waveguides defining optical paths 28A and 28B, at least one of which having a phase modulator in which the refractive index is a function of the strength of the locally applied electric field. In the example of FIG. 1 light in both optical paths 28A, 28B undergoes a phase modulation, although in other configurations the refractive index in only one of the optical paths could be modulated with respect to the other. The beams are then recombined by an output optical combiner 26. Changing the electric field on the phase modulating paths determines whether the two beams interfere constructively or destructively when recombined, and thereby controls the amplitude or intensity of the exiting light. In some configurations, the phase of the exiting light can be controlled via a variety of means such as by manipulating the phase modulation signal, or through design.
In the configuration shown in FIG. 1, the modulating electric field is provided by a segmented Travelling Wave Electrode 21 (or TWE) that consists of two or more transmission line conductors 30A, 30B oriented substantially parallel to the optical paths 28A, 28B, and a plurality of pairs of waveguide electrodes 32A, 32B. Each waveguide electrode 32A, 32B is connected to one of the transmission line conductors 30A, 30B via a corresponding tap or bridge conductor 34A, and 34B. Each bridge conductor 34A, 34B branches out of one of the transmission line conductors 30A, 30B in a direction substantially perpendicular to the optical paths 28A, 28B. The transmission line conductors 30A, 30B convey an RF signal along an RF path that is substantially parallel to the optical paths 28A, 28B.
The configuration shown in FIG. 1 is known as a Mach-Zehnder modulator operated in “push-pull” mode is referred to as a series push-pull travelling wave electrode, after Klein et al., “1.55 μm Mach-Zehnder Modulators on InP for optical 40/80 Gbit/s transmission networks”, OFC/NFOEC 2006, paper TuA2, and described in further detail by R. G. Walker, “High-Speed III-V Semiconductor Intensity Modulators”, IEEE J. Quant. Elect., vol. 27(3), pp. 654-667, 1991. In a series push-pull configuration, a single voltage signal or field is used to phase modulate the interfering signals in the two arms in anti-phase. Each pair of waveguide electrodes 32A, 32B, as shown in FIG. 1, impart a phase change to the optical wave in the waveguide 28A, 28B and also act as a pair of capacitors in series and as a load on the main transmission line conductors 30A, 30B.
A travelling wave electrode Mach-Zehnder optical modulator can be driven using either a single RF signal input, or two RF signal inputs in anti-phase. Referring for example to FIG. 1, in a single-ended design the two transmission line conductors 30A, 30B may respectively act as a signal transmission line conductor (S) conveying the input electrical signal, and a ground transmission line conductor (G) connected to a ground reference. This modulator configuration is sometimes referred to as an SG modulator (also known as a coplanar strip). It should be noted that other types of RF drives are known in the optical telecommunications industry, requiring other arrangements of transmission line conductors in the modulator. For example, the prior art includes optical modulators with differential-drive GSGSG and GSSG formats (see for example applicant's U.S. patent application published under number US2013/0209023 (PROSYK) “Mach-Zehnder Optical Modulator Using A Balanced Coplanar Stripline With Lateral Ground Planes”, filed on Feb. 14, 2013).
FIG. 1A (PRIOR ART) is an elevation view of section A of the optical modulator of FIG. 1, showing two pairs of waveguide electrodes 32A, 32B. Each waveguide electrode 32A, 32B extends over a p-i-n junction 36A, 36B, formed within the corresponding waveguide branch. The p-layer 38A, 38B is in contact with the corresponding waveguide electrode 32A, 32B and the n-layer 40A, 40B is in contact with a common conducting backplane 42. The i-layer 39A, 39B contains a series of layers of InGaAsP of varying composition that acts as the waveguiding core. The entire structure extends on a semi-insulating substrate 43. When an instantaneous change is applied in the voltage difference between the transmission line conductors 30A and 30B, a RF current 44 flows from the highly p-doped contact material 38A beneath waveguide electrode 32A, through the corresponding p-i-n junction 36A and the common conducting backplane 42, and up through the opposite p-i-n junction 36B. The direct current (DC) bias voltage of the backplane 42 is typically fixed by an external DC voltage source (not shown). A simplified electrical diagram of this configuration is shown in FIG. 2. The p-i-n semiconductor layers act as capacitors C that are connected in series through the common conducting backplane 42. This series connection halves the required loading capacitance on the main signal transmission line conductor compared to designs with electrically-independent Mach-Zehnder arms, leading to major performance advantages with regards to bandwidth.
Another type of Mach-Zehnder optical modulator known in the art is an in-phase quadrature modulator, or IQ modulator, for example described in Prosyk et al., “Tunable InP-based IQ modulator for 160 Gb/s”, ECOC 2011 post-deadline paper Th.13.A.5. An example of an IQ modulator 80 is shown in FIG. 3 (PRIOR ART). It includes two series push-pull travelling wave Mach-Zehnder modulators 20a and 20b, nested within a parent Mach-Zehnder interferometer. The parent Mach-Zehnder modulator is defined by a parent input optical waveguide 82 which received an input optical signal, a parent beamsplitter 84 splitting the optical signal into first and second parent optical branches 86A and 86B, each hosting one of the Mach-Zehnder optical modulators 20A and 20B, and a parent combiner 88 that recombines the beams outputted by the individual Mach-Zehnder modulators. In contrast to a standard Mach-Zehnder modulators, which typically is operated by switching between two binary data states (e.g., on and off), IQ modulators may be suited to switching between four states, using a signaling format known as quadrature phase shift keying. This format has many advantages over binary signaling schemes, such as an increase in data rate and efficient use of the optical spectrum.
Although series push-pull travelling wave architecture provides many advantages for Mach-Zehnder and IQ modulators, some difficulties may arise with such designs when high optical powers are coupled into the input waveguide. This is particularly true for Mach-Zehnder and IQ modulators fabricated from direct bandgap semiconductors, such as the compound semiconductor indium gallium arsenide phosphide, as used in the modulators of Klein and Prosyk above. Furthermore, the behavior of series push-pull travelling wave Mach-Zehnder modulator and IQ modulators can depend sensitively on the magnitude of the DC bias applied to the p-i-n junctions. Non-uniformity in the backplane voltage can negatively impact performance, such as the amount of optical phase shift per unit change in electrical signal. Also, other electrodes and diodes in the optical path which share the common backplane, such as DC phase control electrodes as described, for example, in FIG. 1(a) of the Prosyk paper, may also suffer in performance and stability.
Accordingly there is a need in the art for improved series push-pull travelling wave Mach-Zehnder modulators which can alleviates at least some of the disadvantages inherent to the prior art.